The invention relates to a method and apparatus for quantitative, depth differential analysis of solid samples with the use of two ion beams, namely by Rutherford backscattering of light ions and by sputter erosion due to bombardment with medium-mass or high-mass ions. The invention is based on the idea that the analytical potential of the Rutherford backscattering technique can be broadened and increased substantially if this method is employed in combination with sputter sectioning of the sample.
The principle and uses of backscattering spectrometry are described in detail in the book by Wei-Kan Chu, James W. Mayer and Marc-A. Nicolet, entitled Backscattering Spectrometry, published by Academic Press, New York, 1978. The nomenclature employed in this book will also be essentially employed in the description of the invention given below. References to relevant equations or paragraphs of the book will be preceded by "Chu et al".
The backscattering spectrometry method employs a beam of fast, light ions (i.e. He.sup.+ or He.sup.2+) which is directed onto a sample. The desired information about the composition of the sample is obtained by measuring the energy spectrum of the primary particles which are scattered into the solid angle .DELTA..OMEGA. around angle .theta.. If an atom of mass M.sub.2 is located at a depth z of the sample, with z being measured perpendicularly to the surface of the sample, a primary particle of mass M.sub.1 and an initial energy E.sub.0, after being scattered off M.sub.2, exhibits the energy E.sub.1 when leaving the sample. This energy can be expressed as follows (Chu et al, .sctn..sctn.3.2.1 and 3.2.2): EQU E.sub.1 =KE.sub.0 -[.epsilon.]Nz (1)
with the stopping cross section factor [.epsilon.] being given by EQU [.epsilon.]=K.epsilon..sub.in /cos.theta..sub.in)=(.epsilon..sub.out /cos.theta..sub.out) (2)
where K is the so-called kinematic factor which, for a given scattering angle .theta. (the angle between the directions of the incoming particle, and the exiting particle after scattering), depends only on the mass ratio M.sub.2 /M.sub.1 (Chu et al, .sctn.2.2 as well as Tables II to V). The value .epsilon. indicates the mean stopping cross section of the sample for the primary particle along its path between the surface and the scattering center. Subscripts `in` and `out` designate the incoming and exiting particle, respectively. The symbols .theta..sub.in and .theta..sub.out signify the angles between the surface normal and the propagation directions of the incoming and exiting particle bundle. N is the density of the sample in atoms/cm.sup.3 (See Chu et al. .sctn..sctn.3.2.l and 3.2.2).
Without other knowledge about the composition of the sample being examined, Equation (1) cannot be solved since it is--even with the knowledge of .epsilon.--an equation with two unknowns, namely K and z.